One reviewer said it 'will shake the world'. Another, in a less flattering vein, called it 'an 800-page polemic'. One year on and the furore surrounding Jung Chang and Jon Halliday's biography of Mao Zedong, Mao: The Unknown Story, shows little sign of abating.

And with the mainland preparing to next month mark the 30th anniversary of the death of the man who founded communist China, the literary public continues to discuss the latest probe into the legacy of one of the formative figures of the last century.

Having sold more than 500,000 in hardback, a behind-the-scenes debate shows no sign of easing, as western Sinologists steadily pick apart the book and question its startling conclusions.

The book has been translated into 25 languages, is about to be released in Chinese, and has rightly or wrongly made Jung one of the world's most influential writers on China - except, of course, in the one country where Jung has said she most wants the book to be published. Apparently read by members of China's Politburo, the work has been labelled 'maximum-level dangerous'.

'Of course it re-ignited the debate over Mao,' says Richard Baum, a China historian at the University of California and moderator of an e-mail chat forum where the book has been heatedly debated. 'This is the first revisionist work on Mao that is credible from a research point of view. [The authors] gathered a lot of research, triggering debate among historians who then ran to their reference books to check the sources.'

Some did not like what they found. 'One would expect careful and responsible analysis of source material in such an iconoclastic work, but Chang and Halliday have taken great liberties in this regard, distorting, stretching and quoting their sources out of context,' wrote University of Western Ontario professor Alfred Chan in an article published in the academic journal Pacific Affairs.

Mao, who died 30 years ago on September 9, has largely escaped the kind of demonisation heaped on the other great dictators of the past century: Hitler, Stalin and Pol Pot. Although both loved and loathed within China, his reputation has been partly shielded by the Chinese government's decision to venerate him under the formulaic banner of 70 per cent good, 30 per cent bad. He is remembered in official propaganda as a great hero for founding the Communist Party, defeating the Japanese, and uniting the country. That much of this is a crass exaggeration is now widely accepted in western scholarship.

Jung and Halliday's book went further, depicting the former Chinese leader as an egomaniac with little ideological backbone. The book even ends with a dramatic description of Mao in his death throes, saying 'his mind remained lucid to the end, and in it stirred just one thought: himself and his power'. They gave Mao a macabre makeover worthy of a Peking opera mask.

Spending more than 10 years trawling through Chinese and former Soviet Union archives, and interviewing many of his former colleagues and associates, the husband and wife authors rejected conventional Sinologist thinking that Mao 'turned bad' sometime around the mid to late 1950s, and instead suggested he was a monster from the beginning.

Questioning many of the accepted tenets of 20th-century Chinese history, they say Mao's conversion to communism was not out of concern for the workers or peasantry, but because he wanted the Russian stipend that came with it. The survival of the Long March, they wrote, was actually the result of an arrangement between Stalin and Chiang Kai-shek. In return for not destroying the communists, Stalin promised not to kill Chiang's son who he was holding hostage in Moscow. And the Luding Bridge incident, during which communist soldiers crawled along 'incandescent chains' while under heavy machine gun fire, is total fabrication, they claimed.

It is the descriptions of Mao's character, though, that have Sinologists excited. One of the few remaining westerners to have spent time with Mao, Sidney Rittenberg, said he found the book a 'caricature'. Joining the chairman in his Yanan mountain base in the mid-1940s, Mr Rittenberg signed up to the Communist Party, and spent the next 35 years in China, some of that time in prison on charges of being a spy for the US. Although saying he often found Mao 'aloof' and 'not one of the boys', Mr Rittenberg rejected the book's suggestion that Mao's motivation lacked any ideological basis and described how he perceived the deterioration of Mao over the years.

'The thing that impressed me about him when I first met him in the mountains, apart from a great mind, was that he was a great listener. I was an unknown young American who did not know which end was up. And he would listen to my answer as though what I was saying was the most important thing in the world to him ... but later when I was translating his works in the early 1960s he was quite different. When you talked with him he held forth. He did not pay much attention to other people's views. That was a fundamental change. It's the old adage about the corruption of power.'

Meeting in their Notting Hill home in London, where East Asian curios jostle for attention alongside dozens of copies of Mao, Jung and Halliday said that one year on, they felt their conclusions had been vindicated.

'Since the book was published, we have had a lot of response from people all over the world confirming what we wrote with their own sources,' said Halliday. One example, he said, was in the descriptions of the countdown to the Sino-Indian war of 1962. After digging through the Soviet archives they detailed how Chinese and Soviet leaders made an arrangement by which Khrushchev would support Mao's excursions into India if Mao supported the deployment of nuclear missiles in Cuba. 'I think this was a very important moment and nobody had made this point before, but as a result a number of Indian sources have come out and said they knew this was what the Chinese were up to,' said Halliday.

First making her name with Wild Swans, which poignantly depicted how three generations of women in her family survived the turmoil of 20th-century China, Jung says she was shocked by the venom with which some attacked her, including suggestions the book was a deliberate attempt to get even for her family's suffering during the Cultural Revolution. She said they both approached the book with an open mind and it was the research they uncovered that shaped their conclusions.

Accusing some of her detractors of being apologists for Mao, an exasperated Jung said critics couldn't accept that a man who was famously feted by western statesmen in the last years of his life was an absolute tyrant.

'These so-called China experts - these people have written books around those lies, and have taught students around those lies, and they don't want their lie challenged. Sometimes, I think people don't treat Chinese lives with the same weight as other lives. People tend to think there are an awful lot of Chinese. That tens of millions of people perished under Mao is a fact. But that is not enough to make people condemn him?'

It is a viewpoint that some have sympathy for. 'There are plenty of people now flinging mud at Jung and Halliday, who in the late 1960s and early 70s were singing a different tune,' said Mr Baum in reference to earlier western scholarship that praised the Maoist experiments only to discover later they had been catastrophic failures. 'People have looked at China through various subjective lenses for some time and you have to refine your views.'